linux-kernel - Re: [PATCH v6 06/11] mm, compaction: more reliably increase direct compaction priority

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Date:	Thu, 18 Aug 2016 11:44:00 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 06/11] mm, compaction: more reliably increase direct
 compaction priority

On 08/18/2016 11:10 AM, Michal Hocko wrote:
> On Wed 10-08-16 11:12:21, Vlastimil Babka wrote:
>> During reclaim/compaction loop, compaction priority can be increased by the
>> should_compact_retry() function, but the current code is not optimal. Priority
>> is only increased when compaction_failed() is true, which means that compaction
>> has scanned the whole zone. This may not happen even after multiple attempts
>> with a lower priority due to parallel activity, so we might needlessly
>> struggle on the lower priorities and possibly run out of compaction retry
>> attempts in the process.
>>
>> After this patch we are guaranteed at least one attempt at the highest
>> compaction priority even if we exhaust all retries at the lower priorities.
>
> I expect we will tend to do some special handling at the highest
> priority so guaranteeing at least one run with that prio seems sensible to me. The only
> question is whether we really want to enforce the highest priority for
> costly orders as well. I think we want to reserve the highest (maybe add
> one more) prio for !costly orders as those invoke the OOM killer and the
> failure are quite disruptive.

Costly orders are already ruled out of reaching the highest priority 
unless they are __GFP_REPEAT, so I assumed that if they are allocations 
with __GFP_REPEAT, they really would like to succeed, so let them use 
the highest priority.